1. Field of the Invention
This invention relates to the production of silicon for use in solar cells and more particularly to a process for removing various impurities whereby metallurgic grade silicon can be refined to produce silicon for use in solar silicon photovoltaic cells.
2. Description of Related Art
It has long been an objective to convert solar energy into electricity. The leading candidate for implementing this technology is a direct conversion of solar energy to electricity by means of silicon photovoltaic cells. The technology has advanced to a level at which solar silicon photovoltaic cells are available that could provide a viable alternative source of electricity.
However, to date solar silicon photovoltaic cells are costly and are not yet available at a commercially economical basis for producing electricity competitively with the electric grid. Consequently solar energy systems remain too costly and, at present, do not constitute a cost-effective alternative to primary oil, coal, natural gas and propane power generating systems.
For purposes of understanding this invention, commercially available silicon is available in three forms or grades, each being characterized by different levels of impurities and manufacturing costs. Metallurgical grade silicon (hereinafter “MG-Si”) has impurities in the range of 10,000 ppmw and has the least restrictive limits on impurities. It also is the least expensive to purchase, being available for about $3/kg at current prices. Electronic grade silicon (hereinafter “EG-Si”) has the most restrictive limits on impurities, in the range of 1 ppbw. EG-Si therefore is also the most expensive to purchase. Selling prices for EG-Si in the past have been up to $150/kg; current selling prices can be up to triple that amount.
Solar silicon (hereinafter “SoG-Si”) has impurity limits in the range of 1 ppmw and is currently available at a price of about $75 to $250/kg. In order for the solar energy systems to be commercially competitive alternative power sources for the electric grid, it is estimated that the cost for SoG-Si must be reduced significantly; e.g., to about $30/kg.
It now is recognized that boron, phosphorus, iron and aluminum are four impurities that present major obstacles to the efficient production of SoG-Si. Although there are yet no official standards, there seems to be a goal to produce silicon with impurities of boron, iron and phosphorus as follows:
TABLE 1SoG-Si Impurity TargetsIMPURITYPPMWB<0.5P<0.5Fe<1.0Al<0.5
One early proposal for producing SoG-Si is found in WO90/03952 to Schmid et al. that describes a method for growing silicon ingots using a rotating melt. The object of the invention is to produce photovoltaic grade silicon using the heat exchanger method. The disclosed method includes four processes, namely: (1) vaporization of impurities enhanced by vacuum operations in a silica crucible; (2) scavenging a reaction of impurities by slagging with silica and gas blowing with moist hydrogen and/or chlorine; (3) segregation of impurities enhanced by controlled directional solidification; and (4) centrifuging of insoluble particles. The system operates with a vacuum of less than 30 torr with a 0.1 torr vacuum being optimal. The resulting material is still expensive primarily because there are requirements for multiple production processes.
U.S. Pat. No. 4,094,731 discloses an apparatus and a process for producing silicon having a reduced iron concentration. The apparatus incorporates a carbon crucible, a carbon rod stirrer, a nitrogen gas injector and a ladle for decanting a mother liquor before the mixture reaches its eutectic temperature. Motion between a mold wall with the growing silicon crystals and the molten mother liquor continuously washes the exposed growing surfaces of the silicon crystals with the mother liquor. Canting the mother liquor before reaching the eutectic temperature leaves a hollow, ladle-shaped silicon ingot of about 60% of the weight of the original mother liquor and having outer and inner zones. The outer and inner zones are discarded to leave an annular crystalline portion with reduced iron concentration.
U.S. Pat. No. 4,124,410 discloses a process for reducing the level of iron and aluminum impurities. In this process essentially iron-free silicon platelets are precipitated from a solution of MG-Si in molten aluminum. The process next melts the refined platelets in contact with a silica slag and directionally solidifies the refined silicon-slag melt. One or more melts may be used to form a final product.
U.S. Pat. Nos. 4,246,240 and 4,256,717 disclose still another process for reducing iron impurities. This silicon purification process extracts heat from a molten silicon-rich material to provide a solid phase containing silicon in crystal form and a molten phase with concentrated impurities. The molten phase is separated from the solid phase. The solid phase is then remelted to remove the solvent metal, including impurities, from the crystals. At least one fraction of the remelted material is separated from the crystals. The metals or interest are tin, zinc, aluminum, silver and lead. This patent recognizes problems with removing phosphorous and proposes to reduce the level by treating the silicon rich alloy in the molten state with a source of chlorine, such as Cl2, COCl2 and CCl4.
One general approach, as described in International Publication No. WO 2007/127126 filed by Lynch et al in 2006, is designed to remove boron and phosphorus during the conversion of MG-Si to EG-Si. Specifically, the Lynch reference describes a process by which aluminum and fluxing agents (Al2O3, SiO2, CaO and MgO) are added to molten silicon to create an oxy-nitride slag. This slag is stated to act as a sink for dissolved boron and phosphorus. Nitrogen is bubbled through the molten silicon. Aluminum can be added as aluminum metal or as Al2O3. Normally, the silicon must initially be deoxidized to allow boron and phosphorus refining reactions to occur. The process may be followed by oxidative refining, SiC settling, the Silgrain process, and directional solidification to remove other impurities and produce silicon for use in solar cells. In an alternative version of the process, the molten silicon is passed through a particulate bed formed of a nitrogen-containing compound and an aluminum-containing compound.
To the extent that each of the foregoing processes may produce SoG-Si with acceptable levels of impurities, each is complex and expensive to implement. Consequently, the manufacturing cost for the SoG-Si material exceeds the price goal which would enable the production of commercially viable silicon for use in solar silicon photovoltaic cells. What is needed is a process that converts MG-Si into SoG-Si with a manufacturing cost that would enable a construction and operation of solar photovoltaic electricity generating systems to be a commercially viable alternative to conventional electrical energy delivered to the electric grid.